fsmonitor-ll.h: split this header out of fsmonitor.h

fsmonitor-ll.h: split this header out of fsmonitor.h

This creates a new fsmonitor-ll.h with most of the functions from
fsmonitor.h, though it leaves three inline functions where they were.
Two-thirds of the files that previously included fsmonitor.h did not
need those three inline functions or the six extra includes those inline
functions required, so this allows them to only include the lower level
header.

Diff best viewed with `--color-moved`.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>